The present invention relates to a hydraulic servo device with a built-in accumulator.
In hydraulic control systems for automatic transmissions, an accumulator is used to dampening a rise in hydraulic fluid pressure and the like. However, the accumulator piston is large in diameter and needs a long stroke, thus demanding a relatively large space. A hydraulic servo device with a built-in accumulator as shown in FIG. 1 is known which has an accumulator arranged in the same place as a servo piston for actuating a band brake in order to provide a compact arrangement as one measure to solve the above problem. This hydraulic servo device is used in an automatic transmission called "Hydra-matic 400" manufactured by GM. A servo piston 12 has an outer diameter portion fit in a large diameter inner diameter portion 10 of a servo cylinder and an inner diameter portion fit around a large diameter outer diameter portion of an accumulator piston 14. A small diameter outer diameter portion of the accumulator piston 14 is fit in a small diameter inner diameter portion 16 of the servo cylinder. The servo piston 12 is connected to a stem 18. Arranged around the stem 18 are a spring 20 for the accumulator piston 14 and another spring 22 for the servo piston 12. This construction defines a servo apply chamber 24, a servo release chamber 26 and an accumulator chamber 28. A brake adapted to be applied by this servo piston 12 is a band brake for a first speed ratio in L range and thus hydraulic fluid pressure is supplied to the servo apply chamber 24 at the first speed ratio in L range. Thus, no hydraulic fluid pressure is supplied to the servo apply chamber 24 during shifting in D range. Hydraulic fluid pressure is always supplied to the accumulator chamber 28 in D range. Hydraulic fluid pressure is supplied to the servo release chamber 26 at a second speed ratio. Thus, the hydraulic fluid pressure within the servo release chamber 26 gradually rises in shifting from the first to second speed ratio in D range thereby to suppress substantial shift shocks because the accumulator piston 14, which assumes a pressed down position as viewed in FIG. 1, moves upwards as viewed in FIG. 1 owing to the second speed ratio hydraulic fluid pressure supplied to the servo release chamber 26. However, this hydraulic servo with built-in accumulator poses a problem as follows. That is, the accumulator cannot have sufficiently large volume because a pressure acting area of the servo release chamber 26 is relatively small and the accumulator piston 14 is fit in inner diameter portion of the servo piston 12. As a result, a sufficiently effective softening of hydraulic pressure rising cannot be obtained.